Author:

Yongjun Tian(Yanshan University)

The development of novel high-performance superhard materials, guided by
reliable design theories, is highly anticipated for continuous progresses in
processing techniques. In the past decade, we have established the hardness
model for polar covalent single crystals, and revealed an extra hardening
mechanism for polycrystalline materials, which shows a
hardness$-$microstructural size correlation and provides further hardening
at deep nanoscale due to quantum confinement effect. Therefore,
nanostructuring diamond and cBN is still an effective way to enhance
hardness. Based on our model, we estimate the hardness upper limits for
diamond and cBN with nanograined and nanotwinned microstructures,
respectively. Transformed from graphite-like carbon and BN precursors at
high pressure and high temperature (HPHT), nanograined diamond and cBN with
the smallest grain size of $\sim$15 nm can be synthesized, showing
enhanced hardness but reduced thermal stability. Starting from onion-like BN
and carbon, we successfully synthesized nanotwinned cBN and diamond with
average twin thickness of 5 nm or below at HPHT. The simultaneous
enhancements in hardness, fracture toughness, and thermal stability were
confirmed in our nanotwinned cBN and diamond. Our approach offers a general
pathway to nano-structure superhard materials for excellent stability and
ultrahardness, as well as exceptional tradeoff between hardness and
toughness, through the creation of nanotwinned microstructure.

*This work is supported by NSFC under Grants No. 51332005 and No. 51121061.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2014.MAR.B26.8